In the more recent multilamp units such as the "flip-flash" devices, it is desirable to connect the lamps therein in an electrical parallel arrangement. Accordingly, lamp flashing is accomplished in a sequential manner by a series of high voltage pulses impressed across one of the unit's input terminals. In such an arrangement, it is considered highly desirable that the spent (fired) lamp serve as an open-circuit switching element to thus assure that most or all of the subsequent firing pulse will not pass therethrough but instead will trigger the next, unfired lamp. Shorting of lamps as employed in the aforedescribed prior art devices was a ready possibility, however, in that droplets of incompletely burned zirconium or hafnium proved capable of physically bridging the lead-in wires within the lamp's envelope subsequent to lamp ignition. Various attempts to overcome this problem have included utilization of such additional structural parts as insulative glass sleeves (about one or both wires) or the aforedescribed glass or ceramic beads. Not only are lamps using such parts understandably more costly to produce, such lamps are also more difficult to fabricate using today's high-speed lamp manufacturing equipment. In addition, structures such as glass or ceramic beads failed to prove totally effective due to the droplets of conductive material being able on many occasions to settle around the region between the bead and lamp walls and still cause lead-in wire short-circuiting.
As an alternative means of assuring prevention of passage of subsequent firing pulses through a fired lamp, some circuit designs employ photo-sensitive switching members in the form of thin, plastic, metalized webs which each provide conductance to an adjacent lamp. Heat from the flashed lamp melts the web and thus removes the respective lamp from the circuit. The above switching components not only are relatively expensive but also require additional space in the unit's circuit, thus adding to the overall size of the unit.
It is believed therefore that a multilamp photoflash unit which is capable of assuring an open circuit for each fired lamp subsequent to said firing without the need for additional expensive parts as described above would institute an advancement in the art.